This invention relates to a printed wiring board having a special through hole structure suitable for mounting leadless parts. More particularly, the present invention relates to a printed wiring board for mounting leadless parts to provide hybrid IC suitable for highly densified mounting.
With recent progress of technology, parts to be mounted on multi-layer printed wiring boards are reduced in size more and more, and the mounting density of parts becomes higher and higher so as to apply the substrate area at maximum. In order to meet such a requirement, Japanese Utility Model Unexamined Publication No. HI-89779 proposes a multi-layer printed wiring board obtained by laminating an outer layer plate or plates having through holes inner walls of which are plated on one side or both sides of an inner layer plate via prepregs, said through holes being filled with the resin desired from said prepregs to form blind via holes, on which pads for mounting parts are formed. But there are problems in such a structure in that a gas is generated by the heat at the time of soldering parts, spaces are formed between the filled resin and the pads, and the air in such spaces is expanded by the heat to cause misregistration of soldered positions, and the like. On the other hand, there are proposed to fill inner wall metallized through holes of printed wiring boards with solder (Japanese Patent Unexamined Publication Nos. 62-108594, and HI-24491). According to such solder filling processes, there are various problems in that it is very difficult to fill through holes with solder, much care should be taken to handle high-temperature molten solder, various troubles are accompanied to remove solder adhered to unnecessary portions, flying of lead powder produced by polishing the solder is not preferable for health, and the like. In contrast, Japanese Patent Unexamined Publication No. HI-43292 discloses a process for producing a printed wiring board comprising drilling holes in a double-sided copper-clad insulating substrate, filling the holes with an electroconductive paste such as a mixture of a thermosetting resin as a base and a copper paste, or the like, and curing the electroconductive paste by irradiation with electron beams. But since the holes have no electroconductive films on inner walls of the wholes, electric resistance changes easily and thus is unstable, resulting in expecting no stable electric connection. Further, since the electroconductive paste is cured by irradiation with electron beams, there arises a problem of excess curing.